mjhjmtu

Why the following command

Cases[1, 2, 6, 3, 4, 8, 5, x_, y_, z_ /; (y > Max[x, z]):>y]

doesn't return the y values that are greater than their neighbors in the list?
I am expecting to have an output like 6,8.

There are no Lists of length 3 in the input at level 1. So Cases cannot find anything. However SequenceCases can because it interprets lists as sequences.

SequenceCases[
 1, 2, 6, 3, 4, 8, 5, 
 x_, y_, z_ /; (y > Max[x, z]) :> y, 
 Overlaps -> True
 ]

6, 8

ReplaceList with a slight modification of the pattern in OP gives the desired result:

ReplaceList[1, 2, 6, 3, 4, 8, 5, ___, x_, y_, z_, ___ /; (y > Max[x, z]) :> y]

6, 8

The approach proposed in the question works well with a slight modification.

Cases[Partition[1, 2, 6, 3, 4, 8, 5, 3, 1], x_, y_, z_ /; (y > Max[x, z]) :> y]
(* 6, 8 *)

There are many other approaches as well, for instance,

list = 1, 2, 6, 3, 4, 8, 5;
MapThread[If[#1 > Max[#2, #3], #1, Nothing] &, 
 list[[2 ;; -2]], list[[1 ;; -3]], list[[3 ;; -1]]]
(* 6, 8 *)

Timing

Runtime for the MapThread solution with a list of length of 40000 is, for instance,

list = RandomInteger[0, 9, 40000];
AbsoluteTiming[MapThread[If[#1 > Max[#2, #3], #1, Nothing] &, 
 list[[2 ;; -2]], list[[1 ;; -3]], list[[3 ;; -1]]][[1 ;; 10]]]

about 0.06 seconds on my computer and increases approximately linearly with the length of list. The solution using Cases and Partition requires about the same amount of time. Surprisingly, the solution with ReplaceList is about two orders of magnitude slower and increases approximately quadratically with the length of list. The solution using SequenceCases is many orders of magnitude slower and increases at least cubically with the length of list.